1. Field of the Invention
The invention relates generally to programmable read only memories (PROMs) and more particularly to temperature compensated sense amplifiers for detecting the fuse condition of a PROM or similar device.
2. Prior Art
PROMs typically consist of devices, such as transistors, which are joined by fusible links to intersections of rows and columns of wires or buses in an array. The devices may be electrically removed from the row and column array by opening a connecting fusible link, usually done by resistively heating the fusible link until it vaporizes. For example, U.S. Pat. No. 4,042,950 to Price and assigned to the assignee of the present invention, shows a platinum silicide fuse link connecting a Schottky diode and an associated transistor to a conductive bus. Opening or blowing the fuse link disconnects the transistor from the bus.
Prior art sense amplifiers used to sense fuse condition are temperature sensitive. This causes the time between an address change and the resultant output response to that address change to vary at temperature extremes at which semiconductor PROMs are often rated, namely between -55.degree. C. and 125.degree. C.
In the prior art, a switching transistor is typically employed to detect the high or low level of a fuse location, thereby indicating whether or not a fuse has been blown. On and off switching states for the transistor are separated by a threshold voltage between a low level voltage and a high level voltage. Previously at low temperatures, the low level, threshold and high level were widely spaced, causing the time delays mentioned previously, while at high temperatures the three levels tended to converge toward each other thereby reducing noise immunity or causing the devices to sometimes become nonfunctional.